Because animals are more complex than plants, their adaptations are more varied. They exhibit biochemical responses at the cellular level, physiological response of the whole organism such as modification of the circulatory system, or a behavioral response such as modified feeding habits.
We tend to discuss these responses to individual stresses, but in reality and organism must respond simultaneously to a complex of factors and it's the success of this integrated response that determines the organisms fate. For instance, one could move out of the stress, but in a wetland, that might mean moving from an anoxic zone within the soil to the surface where temperature extremes and dessication become a problem.
Evolution has put a premium on aerobic metabolism so the more evolved animals have less ability to adapt to anaerobic conditions than primitive animals. It is the internal cell environment that is closely regulated so most adaptations are organism level ones to maintain the internal environment.,
Adaptations of marine organisms to control gas exchange:
1. development or modification of specialized regions of the body for gas exchange (gills, for example)
2. mechanisms to improve the oxygen gradient across a diffusible membrane, for example, moving to an oxygen rich environment, or moving water across the gills by cilliary action
3. internal structural changes such as increased vascularization, a better circulatory system, or a stronger heart
4. modification of respiratory pigments to improve oxygen carrying capacity
5. behavior patterns such as decreased locomotor activity of closing a shell during low oxygen stress
6. physiological adaptations including shifts in metabolic pathways and heart pumping rates
Osmoconformers-internal environment follows the osmotic concentration of the external environment--their internal salt levels mimic the external, so they are easily permeable to water and salt. Most simple animals are like this.
Osmoregulators-these control their internal osmotic concentration to maintain levels that may be different from the external environment. This is especially the case with organisms that inhabit the upper intertidal zone. They do this by being less permeable to water and salt, and by having controls. An intertidal marsh crab, for example, is less permeable to water and salt than an aquatic blue crab. Regulatory organs typically include gills and renal organs (the kidneys) which can concentrate and excrete salt (also "crocodile tears")
These saltwater animals that can adapt to different salinities must also be able to adjust when the external environment isn't salty enough--heavy rainfall on a low tide, for example. A freshwater fish tends to gain water because it's internal environment is more concentrated than the external so it is adapted to excrete water. A marine animal in a suddenly less saline environment must either move or be able to respond similarly. Few animals can do this.
Alterations in "normal" flood patterns may help or hurt animals. For example, wood storks have poor fledgling success during dry seasons because the dry ground under their nest trees allows access by raccoons which eat nestlings. On the other hand, wood storks have better feeding success and thus brood success when things are a bit dry because their food gets concentrated into smaller pools, thus making it easier to catch. So ideally, wood storks need high water levels early in the breeding season to protect the nestlings, but lower levels later to make food gathering easier. Not coincidently, this is a common pattern in where wood storks breed.
Many species of reptiles and amphibians and small mammals seek shelter from floods by simply climbing above the high water. Larger species may temporarily move to higher ground.
Ground nesting birds are in trouble if flooding occurs during nesting season, but some will simply renest.
Some specifics about fish:
Most wetland fish are dark and drab looking with stripes of bands or mottling. This way they blend in nicely with tannin stained waters and leaf litter.
In flowing streams, fish have no trouble getting enough oxygen, but a still wetland may have very low oxygen levels. Fish rarely suffocate though and may do any of the following:
1. move to alternate microhabitat
2. have tolerance for anaerobic metabolism
3. change their activity levels
4. be able to extract oxygen at very low concentrations
5. increase their breathing at the water's surface
6. increased breathing of atmospheric air (a dorsally flat shape with a superior mouth allows this)
Some fish (gars, bowfins, mudminnows) can absorb air from their air bladders.
Wetland fish tend to be more temperature tolerant than other fish.
Eggs are generally placed in shallow water, often in nests where they are fanned, guarded, or rolled around. All these are thought to be adaptations to low oxygen concentrations. In addition, several species of wetland fish are live-bearers which may also be an adaptation to the variable conditions in a wetland.
Riparian wetlands are important nursery grounds.
Dessication can be a problem for wetland fish and there are several that can actually survive out of the water for extended periods. Marsh killifish can survive on damp mud for 24 hours, and their eggs can remain viable for 3 months. Bowfin have been found in muddy soil in spheres of dry mud. Dried eggs of gar can be reconstituted and hatched. A lot of small fish probably use crayfish burrows to survive in when things get dry.
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